The present invention is directed to a method and to a device for aligning a print head in relation to the material to be printed.
In the course of placing images on offset printing plates, it is known to use multiple head printing systems which have a plurality of ink jet print heads. The print heads spray the appropriate medium on the printing plate, which is customarily mounted on a printing plate cylinder and which moves in the course of the printing process. In order to achieve a printed image of high quality on the printing plate and ultimately on the printed product, it is necessary to align the print heads with an accuracy of a few xcexcm in respect to each other. However, conventional print heads do not have surfaces with which they could be aligned by the use of stops. To position such print heads on a desired spot, it is therefore necessary to compare the actual position of the print image with the intended position and to displace the print heads accordingly.
GB 2 349 607 A shows a printing device with a plurality of print heads which are arranged offset in respect to each other. These print heads are arranged inclined in the radial direction in relation to a printing cylinder.
U.S. Pat. No. 6,048,048 discloses a printing device with several print heads. These print heads are aligned with each other transversely to the printing direction.
The object of the present invention is directed to providing a method and a device for the alignment of print heads.
In accordance with the present invention, this object is attained by changing the shot or actuation time of the print head. To accomplish the alignment of several print heads in respect to each other at least one of the print heads is moved relative to another of the print heads in a direct transverse to the printing direction. A time control device is usable to control the timing of the shot or actuation of the print head to align the print head in the printing direction.
The advantages which can be obtained by the present invention lie, in particular, in that, instead of displacing the print head in two axes for appropriately adjusting the print image sprayed by the print head on the material to be imprinted, such as an offset printing plate, the alignment of the print head, or of its resultant print image, can be achieved more simply, in accordance with the present invention, by a combination of a mechanical alignment in one axis and by a time control of the shot of the print head. Thus, for aligning the print head transversely with respect to the printing direction, the print head is moved, while for aligning the print head in the printing direction, the time of the shot from the print head is changed. The term print direction in this context is understood to be the movement direction of the material to be imprinted relative to the print heads. If printing plates mounted on rotating printing plate cylinders are provided as the material to be imprinted, the print direction extends perpendicularly or vertically with respect to the axis of rotation of the printing plate cylinder. The change in the time of the shot is usefully made as a function of the print speed, i.e. as a function of the running speed of the material to be imprinted. If the time of the shot from the print head is delayed, the material to be imprinted passes farther underneath the print head and vice versa, so that an adjustment of the position of the printed image on the material to be imprinted, in the printing direction, is achieved.
Several print heads of a multiple head printer can, in particular, be aligned with each other, wherein at least one of the print heads can be displaced for the alignment of the print heads transversely to the printing direction. To accomplish the alignment of the print heads in the printing direction, the time of the shot from at least one print head can be controlled. The displacement of the print head, as well as the change of the time of the shot, can be performed individually for each print head, or possibly can also be performed for groups of print heads together.
The print heads are, in an advantageous manner, individually displaced. They are moved exclusively for achieving an alignment of the printed image transversely to the printing direction. The adjustment of the printed image is considerably simplified by this. When performing a displacement of the print head in only one direction, it is not necessary to keep in mind that a displacement of the printed image in the other axis could possibly also occur. The latter is adjusted by the timed control of the time of the shot.
Preferably, all of the print heads which are displaced, are displaced in the same direction, i.e. along parallel axes. This simplifies the alignment of the print heads in relation to each other.
The print heads can be displaced transversely to the printing direction. In this way, the alignment transversely to the printing direction is disassociated from the alignment in the print direction, so that the alignment process as a whole is simplified.
In accordance with another preferred embodiment of the present invention, the print heads can each be moved, or pushed along an axis which forms an acute angle in respect to the printing direction. A resultant offset of the print heads in the printing direction can be compensated for by a corresponding change of the time of the shots from the respective print head. In connection with obliquely placed print heads in particular, i.e. print heads, whose nozzle row was placed obliquely in relation to the printing direction in order to permit a closer arrangement of the print heads in relation to each other, the displacement of the print heads can take place parallel with the oblique position, i.e. parallel in relation to the longitudinal orientation of the print heads defined by the row of printing nozzles. It is possible by such oblique placement in an arrangement of the print heads in several rows, for print heads of different rows to be fastened on a mutual, obliquely placed print head holder. A displacement can be performed in the longitudinal direction of the holder connecting the print heads.
In an advantageous manner, an adaptation of the printing grid distance in the printing direction to the printing nozzle distance transversely to the printing direction can be performed by the time control of the individual nozzle shots as a function of the printing speed.
The alignment can be achieved, in an advantageous manner, by the use of a print head positioning device which is usable for exclusively aligning the print head transversely to the printing direction. A time control is used for controlling the time of the shot of the print head for aligning it, and the resultant printed image, in the printing direction.
The positioning device is preferably a single-axis displacement device, by use of which the print head can be displaced along one axis. The displacement axis of the positioning device can be arranged transversely to the printing direction. In accordance with a preferred embodiment of the present invention, the print head positioning device has a displacement axis which forms an acute angle with the printing direction. The time control device is embodied in such a way that the resultant offset of the print head in the printing direction is compensated for by a change of the time of the shot, at which shot time, the respective print head shoots the individual drops.
In a further development of the present invention, the device has a plurality of print head positioning devices for aligning several print heads in relation to each other. The print heads are preferably adjustable individually and separately from each other. The control device can be configured in such a way that the shot time, or times, of the print heads can be set individually and/or in groups.